Wireless communications systems are widely deployed, providing a variety of data and voice communication services to mobile subscribers. In Code Division Multiple Access (CDMA)-based systems, e.g. WCDMA, individual channels are formed by frequency-spreading individual communication signals with orthogonal or nearly orthogonal codes, and transmitting a plurality of spread signals simultaneously in the same broad frequency band. A receiver then correlates a received signal with a particular spreading code to recover the corresponding communication signal, and treats all other signals in the band as noise.
In wireless systems, the physical channel between terminals is formed by a radio link. In most cases, many different propagation paths exist between the terminals, due to reflections in the environment. The plurality of propagation paths gives rise to a multipath channel carrying several resolvable components. Because CDMA channels are extracted at a receiver by correlating a received signal with a known spreading code, the receiver performance is improved by utilizing the signal energy carried by many multipath components. This is traditionally achieved by using a RAKE receiver.
The RAKE receiver derives its name from its rake-like appearance, wherein multiple, parallel receiver fingers each receive the multipath signal. Each finger is provided with a reference copy of the spreading code that is delayed equally to the path delay of a corresponding multipath component. The finger outputs are then coherently combined to produce a symbol estimate. In this manner, the RAKE receiver utilizes multipath reception to improve the Signal-to-Noise Ratio (SNR) of the received multipath signal. RAKE receiver performance is optimized if the signal energy from all paths is utilized. To provide the properly delayed spreading code to each finger, the RAKE receiver requires knowledge of the multipath delays and the values of the channel impulse response for all paths.
The multipath delays of the radio channel may be found in a number of different ways. A traditional and efficient solution is based on the power delay profile (PDP) of the multipath channel. The PDP is produced by performing a sequence of correlation operations for each delay of interest, and indicates the detected signal energies at each delay. The PDP may then be inspected to determine the exact delays of the physical paths, possibly after additional processing, so that these delays may be provided to RAKE fingers. A known way to generate the PDP is to correlate the received sample sequence with a reference pilot channel (CPICH) chip sequence that is appropriately delayed. Each correlation result produces a PDP value for the given delay.
Destructive interference among multipath signal components gives rise to a phenomenon known as fast fading. For example, if two signal components arrive at an antenna along paths that differ in length by a half wavelength (or multiple thereof), they will be 180° out of phase, and will cancel each other out. Due to fading, the instantaneous PDP—one generated in a single activation period or PDP measuring period—may not indicate the presence of all the physical propagation paths, but only a subset. If the instantaneous PDP were used for RAKE finger placement without further processing, some positions or delay regions containing signal energy may not be accounted for, degrading the RAKE receiver's performance.
In order to avoid ignoring some of the paths, instantaneous PDPs may be averaged with previously calculated PDPS, in order to mitigate the fast fading effects. By using the filtered PDP, all signal paths that are physically present will be considered by the RAKE receiver, even if some of them happen to be invisible during the activation period producing the most recent instantaneous PDP. The averaging time constant must be chosen as a compromise between effectively mitigating fast fading and the ability to track the changes in the underlying path delays. Filtering that is too slow may fail to react to changes in the path profile, such as when a moving mobile terminal turns a corner and transmitted signals encounter a different reflection environment. On the other hand, filtering that is too fast may not sufficiently mitigate fading effects.
In addition to path searching, the PDP is also used for signal power measurements to be reported to the network, in order to aid the network in mobility management, such as handover decisions. The Received Signal Code Power (RSCP) and Received Signal Strength Indication (RSSI) metrics are computed based on the PDP. For the reporting purposes, the instantaneous PDP is used, since the reports must reflect the instantaneous fading state, not the average delay profile properties. Similar PDP computation is also required for the cell search process, when detecting the presence and the signal strength of other cells in the neighborhood of a mobile terminal.
A variety of advanced receiver types have been developed to improve the RAKE receiver performance. One such advanced receiver type is dual-antenna receiver, where a signal is received via two separate antennas, each having a separate RF and receiver front-end processing branch (e.g., DAC, filter, and the like). If the two antennae and receiver branches are sufficiently separated (both spatially and electrically), the fading effects, noise, and interference signal components on the two branches, as seen by the RAKE receiver, are substantially uncorrelated. A dual-antenna RAKE receiver exhibits improved performance due to the array gain (more signal energy is received) and the diversity gain (the probability of deep fades is reduced). As a result, the block error rate performance of the receiver is improved.
Just like the single-antenna receiver, a dual-antenna RAKE receiver requires knowledge of the multipath delays. On a mobile terminal, the receiver antenna separation is sufficiently small that the underlying delay profile from the transmit antenna to both receiver antennae is identical. Therefore, the same path search operation appropriate for a single-antenna receiver could, in principle, be utilized, whereby only one antenna input signal is used to produce the PDP. The long-term averaged, or filtered, version of this PDP would be applicable to both antenna branches. On the other hand, the availability of two (almost) independently faded input signals from the two receive antennas allows the effective probability of deep fades for the paths to be reduced. That is, for a given path to be invisible at a given time instant, it must be faded down at both antennae simultaneously—an outcome having a considerably lower probability than a fade at one antenna only. Accordingly, if both antennae are utilized, heavy filtering of the instantaneous PDP is not necessary and the dynamic tracking of the path delays may be improved.
Several receiver configurations and operating methods are known for generating a delay profile for a dual antenna receiver. As mentioned above, the signal from only one antenna may be used to generate a PDP that is used for signals from both antennae in a dual-antenna RAKE receiver, filtering the PDP to mitigate the effects of fading. This approach has the advantage of requiring only one PDP generation circuit, but requires long-term averaging to produce a fading-independent average PDP. In addition, the instantaneous PDP from the second antenna is not available for the power measurement reports.
An alternative configuration utilizes two parallel PDP generation circuits operating simultaneously, with each generating an instantaneous PDP from the signal received at a different antenna. The two instantaneous PDPs may then be combined to generate a composite PDP. In this arrangement, an instantaneous PDP is available for both antennae for reporting power measurements, and a shorter averaging period is required for the filtered PDP due to the reduced probability of simultaneous deep fading. However, this configuration requires either two separate PDP generation circuits, or a single PDP generation circuit having twice the processing speed.
Still another configuration utilizes two antennae and two receiver front-end processing circuits, and one PDP generation circuit. The PDP generation circuit considers the samples from each antenna alternatively—on either a per-activation period or per-sample basis—and averages the results. This configuration requires only a single PDP generation circuit and reduces the filtered PDP averaging time. However, if the signals are switched on a per-activation period basis, the instantaneous path profile and the signal quality at the two antennae may differ and the input into the PDP averaging does not change smoothly, resulting in fluctuations of the averaged PDP. Additionally, the instantaneous PDP for one antenna will always be out of date. If the signals are switched on a per-sample basis, excessive data loss may occur, as some data is lost in each switching action due to the non-zero delay spread.